Like many metals, the so-called transition metals of the periodic table, such as titanium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten are typically found in nature as oxides, hydrous oxides, or hydroxides. For example, titanium is found naturally as rutile ore, which is preponderantly TiO2. The natural ores of many of these transition metals include additional metals such as lead, iron, calcium, or magnesium. For example, vanadium is primarily obtained from the minerals vanadinite (Pb5(VO4)3Cl) and carnotite (K2(UO2)2VO4.1-3H2O). Niobium is primarily obtained from the minerals columbite ((Fe, Mn, Mg)(Nb, Ta)2O6) and pyrochlore ((Ca, Na)2Nb2O6(O, OH, F)). Various chemical and physical processes may be used to isolate the desired transition metal and purify it as an oxide. However, the chemical reduction of the metal oxides to elemental metal is often difficult. For example, the production of titanium metal from rutile ore is generally accomplished by what is known as the Kroll process, and is described in U.S. Pat. No. 2,205,854 which issued Jun. 25, 1940. This process involves dropping or spraying liquid titanium tetrachloride into molten magnesium to produce titanium metal and magnesium chloride. A variation on this process, called the Hunter process, substitutes liquid sodium for the liquid magnesium, and produces titanium metal and sodium chloride.
In addition to natural or processed ores, metal oxides are also formed on the surfaces of manufactured metal powders. In some cases these oxides are produced deliberately and in other cases, particularly with metal powders, these oxide layers are undesired but occur simply by exposure of the powder to air. This surface oxidation effect is called passivation. It is often desirable to remove these oxide coatings while minimizing the removal or conversion of the underlying metal. Removal of these coatings is challenging because of difficulties that occur separating the mixtures that are created by most chemical removal processes.
In the sixty-some years since the introduction of the Kroll process, many alternative processes have been proposed and some have been patented, but none has replaced the Kroll process to any significant extent. This long history of attempts to replace the Kroll process attests to the need to develop alternative processes that are safer, faster, less expensive and create less waste in the conversion of transition metal oxides, metal oxides and passivated metal powders to elemental metal.